Two-dimensional imaging array of chalcogenide glass bolometers

ABSTRACT

A far-IR imaging array comprising a two-dimensional array of chalcogenide glass thermistor bolometers. A two-dimensional system of metallic strips arranged in a grid structure has insulating film between the strips at the points of intersection of each horizontal and vertical strip. A bolometer is placed at each intersection so as to bridge the insulator thereat. If an infrared image is focused on the array, the resistance of each bolometer can be determined by a scanning system.

United States Patent Bishop et al.

[ Oct. 23, 1973 TWO-DIMENSIONAL IMAGING ARRAY OF CHALCOGENIDE GLASSBOLOMETERS [75] Inventors: Stephen G. Bishop, Arlington, Va.;

William Moorgl-lyattsyille, it/1d. [73 Assignee: The United States ofAmerica as represented by the Secretary of the Navy [22] Filed: June 8,1972 [21] Appl. No.: 260,864

[52] US. Cl. 250/338, 250/349 [51] Int. Cl. G0lt 1/16 [58] Field ofSearch 250/83.3 H, 833R, 250/220 M, 338, 349, 332; 73/193 R, 355 R;

[56] References Cited UNITED STATES PATENTS 3,564,257 2/1971 Berry etal. 250/83.3 H

Primary Examiner-James W. Lawrence Assistant ExaminerDavis L. WillisAttorneyR. S. Sciascia et al.

[57] ABSTRACT A far-1R imaging array comprising a two-dimensional arrayof chalcogenide glass thermistor bolometers. A two-dimensional system ofmetallic strips arranged in a grid structure has insulating film betweenthe strips at the points of intersection of each horizontal and verticalstrip. A bolometer is placed at each intersection so as to bridge theinsulator thereat. If an infrared image is focused on the array, theresistance of each bolometer can be determined by a scanning system.

10 Claims, 1 Drawing Figure TWO-DIMENSIONAL IMAGING ARRAY OFCHALCOGENIDE GLASS BOLOMETERS STATEMENTOF GOVERNMENT INTEREST BACKGROUNDOF THE INVENTION This invention relates to two-dimensional arrays ofindividually addressed radiation detectors and especially to such anarray of far-IR (infrared radiation) detectors.

Many imaging devices exist which operate in the visible andnear-infrared spectral ranges. These devices nearly all employ uniformand continuous photoemissive or photoconductive surfaces for thedetection of the radiation. Such systems do not detect the infraredradiation emitted by bodies at or near room temperature. Imaging systemswhich do operate in this spectral range (wavelengths of 2 to microns)usually employ a linear array of separately addressed infrared detectorsacross which the image to be detected is periodically swept by amechanical scanning system. Each detector has a separate signalprocessing channel. The detectors used may be either cooled extrinsic orintrinsic photoconductors or room temperature thermal detectors such ascrystalline thermistor bolometers or pyroelectric detectors. Each ofthese detectors must be fabricated and mounted separately and electricalwiring connections mustbe provided. The complexity and expense of thisprocess limits the number of detectors provided in a linear array. Theconstruction. of a twodimensional array or x-y matrix of such detectorsis prohibitively expensive. A two-dimensional array eliminates the needfor scanning the image, obviating the need for moving parts in thesystem. Such arrays exist,

but contain so few detectors as to provide poor resolution and areexpensive.

Another disadvantage to the linear array system is that the speed withwhich the image is swept across the detectors determines the requiredspeed or response time for the detectors. In a two-dimensional array,only the movement of the subjects in the image need be accounted for indetermining detector speed or response time. If a relatively stablescene is to be viewed, slow, sensitive detectors may be used therebyimproving image quality.

BRIEF SUMMARY OF THE INVENTION The objects and advantages of the presentinvention are obtained by using glass bolometers as resistive elementsbetween horizontal and vertical, electrically conductive strips. Thestrips are arranged in a rectangularcoordinate grid and the bolometersare located at the intersectional points so as toeonnect between thehorizontal and vertical strips. The strips are electrically insulatedfrom each other.

OBJECTS An object of this invention is to provide a twodimensionalimaging array of individually addressed IR detectors, especially onethat operates in the far-IR region. V

Another object is to provide such an array which is simple to construct,inexpensive and easily interfaced with present-day scanning devices.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a schematicillustration of a two-dimensional array used in an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION As can be seen from the FIGURE,the array comprises a plurality of horizontal and vertical metallicstrips, 10 and 12 respectively, arranged perpendicularly with respect toeach other in a coordinate or grid structure, which will hereinafter becalled an x-y grid. The FIGURE shows a 3 X 3 grid although larger gridsmay be easily constructed.

The metal may, for example, be molybdenum and the array is constructedby depositing the horizontal strips by evaporation or r-f sputteringtechniques on a substrate such as planar glass. Each horizontal strip 10has a vertical-strip appendage 14 at regularly spaced horizontalintervals and in vertical columns. The patterns of the depositions aredefined by standard photolithographic masking techniques.

Pads 16 of an electrically insulating material such as silicon dioxide(SiO are then deposited over each horizontal strip 10 between theappendages 14. Next vertical strips 12 of molybdenum are deposited sothat each passes over a column of the insulating pads 16. Thus, none ofthe vertical strips makes electrical contact with any of the horizontalstrips.

Finally, rectangular or square detector elements 18 are deposited sothat one edge overlaps and contacts a verticalappendage strip 14 and theright edge overlaps and contacts a vertical conducting strip 12.'Thesedetector elements, or thermistor bolometers, 18 are formed from asemiconductor glass, such as the chalcogenide glass TI Se As Te whichchanges its electrical conductivity as a function of its temperature.Infrared radiation causes heating of this glass and consequent change inits conductivity or resistance. The glass operates at room temperatureand at atmospheric pressure. It is sensitive to radiation from thevisible, near IR and Far-IR regions.

There is a large range of alloy compositions in the group of elementsconsisting of thallium (Tl selenium (Se), arsenic (As) and tellurium(Te) from which such thermistor bolometers can be constructed havingvarious sensitivities. The Cd-Ge-As, Cd-GeP, As- Te-Se groups, andgroups based on Ge-Te and containing varying amounts of As, Si and Gealso provide some alloy compositions which are suitable. In general,

any group of amorphous or glassy alloys based on the chalcogens S, Seand Te may provide suitable compositions.

By selecting the proper pair of vertical and horizontal strips, it ispossible to measure the resistance of a given bolometer, since one andonly one bolometer bridges a given pair of vertical and horizontalstrips. If an image or pattern of IR is focused on the array, theintensity pattern of the radiation (i.e., the image) can be determinedor reproduced by measuring the resistance of each bolometer in thearray. This is done by scanning techniques.

Apparatus for scanning the array is described in the literature; forexample, see Chap. 19, Phototelectronic Imaging Devices," Vol. 2, 197i,by Biberman and Nudelman, published by Plenum Press, N.Y.

An array of chalcogenide glass dots or bolometers may be employed as theretina in a vidicon-type electron tube. Such thermicons have beenconstructed using continuous layers of crystalline semiconductors.However, it is difficult to achieve low dark current in such devices. inaddition, lateral heat conductivity in the semiconductor layer causesblurring of the image. With the chalcogenide glass, an array of tinydots or islands of semiconducting material which would be well isolatedfrom one another could be deposited on a thermally insulating substrateby the r.f. sputtering technique. In addition, a relatively lowconductivity glass could be chosen to limit the dark current. Theoperation of this system is similar to that of a standard vidicon inwhich a photoconductive layer with transparent electrode on the frontsurface is scanned by an electron beam on the back surface. In the caseof a thermicon, thermally activated changes of conductivity rather thanphotocurrent are sensed by the scanned electron beam.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

1. A light-imaging array comprising, in combination:

a plurality of conductors arranged in orthogonal,

spaced horizontal rows and vertical columns to form arectangular-coordinate grid, each row being extended to form verticalappendages each of which is spaced from its nearest columnar neighbor;

a plurality of electrically insulative pads, each pad being loacated ata different intersection of the rows and columns of said grid and eachpad lying between the row and column conductor at its associatedintersection so that the rows and columns do not make electrical contactwith each other; and plurality of thermistor bolometers fabricated fromsemiconductor material whose resistance changes as a function of itstemperature, each bolometer being located at a different intersection ofsaid grid, and each bolometer lying across and being in electricalcontact with the vertical appendage and the columnar conductorassociated with its intersection.

2. An array as in claim 1, said semiconductor material being operativeat room temperature and at atmospheric pressure.

3. An array as in claim 1, said semiconductor material being sensitiveto light in the far-IR region.

4. An array as in claim 1, said semiconductor material being of thechalcogenide glass type.

5. An array as in claim 1, said semiconductor material being of glassformed from an alloy based on the chalcogens S, Se and Te.

6. An array as in claim 1, said semiconductor material being of glassformed from an alloy based on the group As, Te and Se.

7. An array as in claim 1, said semiconductor material being a glasshaving the composition Tl Se As Te 8. An array as in claim 1, saidsemiconductor material being a glass based on the GeTe elements andcontaining an amount of As or Si.

9. An array as in claim 1, said semiconductor material being a glassbased on the Ge-Te elements and containing various quantitativecombinations of As and Si.

10. An array as in claim 4, wherein said conductors are in the form ofmetallic strips.

2. An array as in claim 1, said semiconductor material being operativeat room temperature and at atmospheric pressure.
 3. An array as in claim1, said semiconductor material being sensitive to light in the far-IRregion.
 4. An array as in claim 1, said semiconductor material being ofthe chalcogenide glass type.
 5. An array as in claim 1, saidsemiconductor material being of glass formed from an alloy based on thechalcogens S, Se and Te.
 6. An array as in claim 1, said semiconductormaterial being of glass formed from an alloy based on the group As, Teand Se.
 7. An array as in claim 1, said semiconductor material being aglass having the composition Tl2Se As Te3.
 8. An array as in claim 1,said semiconductor material being a glass based on the Ge-Te elementsand containing an amount of As or Si.
 9. An array as in claim 1, saidsemiconductor material being a glass based on the Ge-Te elements andcontaining various quantitative combinations of As and Si.
 10. An arrayas in claim 4, wherein said conductors are in the form of metallicstrips.